This invention relates to a differential amplifier circuit and, in particular, to a differential amplifier circuit capable of preventing occurrence of noise.
Following recent development of a multi-functional high-speed electronic system apparatus, a semiconductor device is systemized and developed towards a larger scale and a higher speed with a CPU and a memory device built therein. In order to adapt the semiconductor device to a large-scale high-speed electronic system, various techniques are adopted in the semiconductor device. For example, by lowering a power supply voltage and stepping up or down the power supply voltage inside the semiconductor device as desired, power saving and a higher speed are achieved. For a higher speed, various approaches are made, for example, a reduction in signal amplitude, adoption of complementary data, and an expansion of a data bus width.
However, the reduction in signal amplitude for the purpose of a higher speed poses a new problem. When the signal amplitude is reduced, complementary signals and a differential amplifier circuit for comparing a signal potential and a reference potential are frequently used. However, such differential amplifier circuit is disadvantageous in that an operation speed is decreased and an operation error is caused to occur due to small variation in input potential resulting from noise. Referring to FIG. 1, the differential amplifier circuit will be described. The differential amplifier circuit comprises differential input transistors 1 and 2, current mirror transistors 3 and 4, current source transistors 5, 6, and 7, a pull-up transistor 12, output transfer gate transistors 13 and 14, clocked inverter circuit transistors 15, 16, 17, and 18, and a plurality of inverter circuits.
The differential input transistor 1 has a drain, a source, and a gate connected to a node N9, a differential common node N3, and an input signal terminal IN1, respectively. The differential input transistor 2 has a drain, a source, and a gate connected to a judgment output node N4, the differential common node N3, and an input signal terminal IN2, respectively. The current mirror transistor 3 has a drain, a source, and a gate connected to the node N9, a power supply potential, and the node N9, respectively. The current mirror transistor 4 has a drain, a source, and a gate connected to the judgment output node N4, the power supply potential, and the node N9, respectively. The gates of the current mirror transistors 3 and 4 are connected to the drain of the differential input transistor 1 to form the node N9. The drains of the current mirror transistor 4 and the differential input transistor 2 are connected in common to form the judgment output node N4.
The current source transistor 5 has a drain, a source, and a gate connected to the differential common node N3, a drain of the current source transistor 6, and a node N5, respectively. The current source transistor 6 has the drain, a source, and a gate connected to the source of the current source transistor 5, a drain of the current source transistor 7, and the power supply potential, respectively. The current source transistor 7 has the drain, a source, and a gate connected to the source of the current source transistor 6, a ground potential, and the power supply potential, respectively. When the current source transistor 5 is turned on and the differential common node N3 is given a low potential, input signals are subjected to differential amplification.
The pull-up transistor 12 has a drain, a source, and a gate connected to the judgment output node N4, the power supply potential, and the node N5, respectively, and pulls up the judgment output node N4 when differential amplification is not operated. The transfer gate transistor 13 has a drain, a source, and a gate connected to a node N10, a node N11, and the node N5, respectively. The transfer gate transistor 14 has a source, a drain, and a gate connected to the node N10, the node N11, and an inverted signal of the node N5, respectively. When the differential amplification is operated, an output transfer gate (13 and 14) is in a conductive state and delivers, to the node N11, a signal sent from the judgment output node N4 through the inverter circuits. During a non-operating period without carrying out the differential amplification, the output transfer gate is in an interrupted state and interrupts the signal from the judgment output node N4.
The clocked inverter circuit transistors 15, 16, 17, and 18 form a clocked inverter circuit INV19 and, in combination with one inverter INV14 in an output path, form an output latch circuit for latching output data. The clocked inverter circuit transistor 15 has a drain, a source, and a gate connected to a source of the clocked inverter circuit transistor 16, the power supply potential, and the node N5, respectively. The inverter circuit transistor 16 has a drain, the source, and a gate connected to the node N11, the drain of the clocked inverter circuit transistor 15, and a node N12, respectively. The clocked inverter circuit transistor 17 has a drain, a source, and a gate connected to the node N11, a drain of the clocked inverter circuit transistor 18, and the node N12, respectively. The clocked inverter circuit transistor 18 has the drain, a source, and a gate connected to the source of the clocked inverter circuit transistor 17, the ground potential, and an inverted signal of the node N5, respectively.
During operation of the differential amplification, the clocked inverter circuit transistors 15 and 18 are in an off state and the clocked inverter circuit has a high impedance. In this case, the output latch circuit does not carry out a latch operation. Therefore, the inverter circuit INV14 delivers data to an output side. When the differential amplification is not operated, the inverter circuit iNV14 and the clocked inverter circuit INV19 are loop-connected to form the output latch circuit which carries out the latch operation. The inverter circuits INV1 to INV18 are used to generate a control signal obtained by inverting a signal supplied thereto and to control an operation timing by delaying the control signal via connection of a plurality of stages.
In the above-mentioned differential amplifier circuit, when a start signal V1 is turned from a “H” level into a “L” level, the pull-up transistor 12 is turned off and the pull-up operation is stopped. The current supply transistor 5 is turned on and the differential common node N3 is given a low potential so that the differential amplification is started. Potentials of input signal terminals IN1 and IN2 are compared and judged and a result of comparison and judgment is reflected at the judgment output node N4. When the start signal V1 has a “H” level, the current source transistor 5 is turned off and the differential amplification is not carried out. The judgment output node N4 is given a “H” level. However, the output transfer gate is interrupted so that the output latch circuit latches a judgment output of a previous cycle and delivers the judgment output of the previous cycle to an output terminal N8.
However, when the current source transistor 5 is turned on and the differential common node N3 is dramatically lowered in potential, the potentials of the input signal terminals IN1 and IN2 are also lowered therealong due to coupling between the sources and the gates of the differential input transistors 1 and 2. At this time, due to a difference between or a state of floating capacitances (C3 and C4) of input nodes N1 and N2, variation in potential is different. Generally, wiring lengths from the input signal terminals IN1 and IN2 are different so that capacitance values C3 and C4 are different. In this case, the input nodes N1 and N2 are different in potential change. The “H” level may significantly be lowered below the “L” level of the input node on an opposite side. Due to the change in potential difference, comparison and judgment is not correctly performed so that the differential amplifier circuit causes an operation error. In particular, when the signal amplitude is small, a potential difference between two inputs is small and an operation error tends to frequently occur.
As the differential amplifier circuit started by a control signal, a sense amplifier is known. The sense amplifier is disclosed in the following patent documents. Japanese Unexamined Patent Application Publication (JP-A) No. H5-342854 (Patent Document 1) discloses an open-bit sense amplifier in which a transfer gate controls connection of bit lines to perform high-speed sensing. Japanese Unexamined PatentApplication Publications (JP-A) Nos. 2000-82290 (Patent Document 2) and H7-105693 (Patent Document 3) disclose a sense amplifier in which a transfer gate controls connection of bit lines to achieve a high-speed operation of the sense amplifier.
Japanese Unexamined Patent Application Publication (JP-A) No. H5-342871 (Patent Document 4) discloses a sense amplifier circuit in which an output of a sense amplifier is selectively delivered to a main amplifier by the use of a column switch. Japanese Unexamined PatentApplication Publication (JP-A) No. 2001-185999 (Patent Document 5) discloses a sense amplifier circuit in which a dynamic NOR circuit is provided at a preceding stage of an amplifier and produces an output for controlling a transfer gate. Japanese Unexamined Patent Application Publication (JP-A) No. 2001-307494 (Patent Document 6) discloses a semiconductor memory in which capacitance loads connected to a sense amplifier are switched and equalized by an address signal to improve an operation margin.
The above-mentioned patent documents are all related to the sense amplifier. In the patent documents 1 to 5, the transfer gate is controlled in order to remove noise from an adjacent bit line or to perform high-speed sensing by isolating a bit-line capacitance. The wiring lengths of the bit lines connected to the sense amplifier are equal to one another and wiring capacitances are equal to one another. In such sense amplifier, an input signal and an output signal are directly coupled and an input parasitic capacitance (load capacitance) is sufficiently large. Therefore, the problem addressed by this application is not posed. The patent document 6 aims to equalize the wiring capacitances in case where the number of banks is different in a ROM. The problem addressed by this invention, i.e., that the potential of the input signal is varied and a differential amplifying operation is not correctly carried out because the potential of the differential common node dramatically decreases at the start of the differential amplifying operation, is not described in these patent documents. None of the patent documents is conscious of the problem addressed by this invention and does not suggest the solution therefor.
In the semiconductor device, the signal amplitude is reduced to achieve a high-speed operation. When the signal amplitude is reduced, complementary signals and the differential amplifier circuit for comparing a signal potential and a reference potential are used. However, as described above, the differential amplifier circuit is disadvantageous in that, because the potential at the differential common node is dramatically lowered at the start of the differential amplifying operation, the potential of the input signal is varied and the differential amplifying operation can not correctly be carried out.